This invention relates to a hydraulic system controlling the displacement of variable displacement pumps.
In the hydraulic system in which a variable displacement pump is driven by a prime mover such as a diesel engine, it is essentially required to make arrangement so as not to impose an excessive load on the prime mover and utilize the output of the prime mover effectively against a wide range of loading pressures.
For this reason, the displacement of variable displacement pump is controlled in such a manner as to match the total input torque of a plurality of variable displacement pumps simultaneously driven by the same prime mover with the rating output torque of the prime mover regardless the loading pressure of each of the variable displacement pumps.
In brief, the following relative formula is obtained between the delivery pressures and displacement of variable displacement pump and the rating output torque thereof. ##EQU1##
Where Pp, Pp* . . . are delivery pressures of variable displacement pumps, Q and Q* are displacements thereof and Tr is rating output torque thereof.
Heretofore, in order to effect the displacement control in a simple manner to satisfy the above formula, displacement controlling devices of a plurality of variable displacement pumps have been mechanically connected and approximately non-linear control given by the above formula has been made by multiple springs.
In this case, because flow rates of the fluid delivered by all variable displacement pumps are set always equal, when controlling actuators at extremely slow speeds or preventing generation of excessive pressures, an excessive volume of fluid is ineffectively delivered by the pumps. Therefore, a considerable amount of power has been wasted as heat losses.
Such unavoidable heat losses are attributable to mechanical connection of the displacement controlling devices of a plurality of variable displacement pumps.
Stating in brief, such heat losses can be avoided by rendering it possible to control the displacements of variable displacement pumps individually, allowing a necessary amount of fluid to be deliverred and controlling the maximum displacement or volume of the fluid to be deliverred by the pumps. EQU Q.ltoreq.Q.sub.max, Q*.ltoreq.Q.sub.max, ##EQU2##
The prior art devices employing multiple spring are liable to be subjected to external disturbances and have a big mechanical hysteresis, and so it has been difficult to achieve effective utilization of power.
For this reason, there is an embodiment of such device in which the displacements of individual variable displacement pumps are set by a servo type displacement varying device and the relationship represented by the aforementioned formula can be obtained approximately by controlling the pilot pressure of the displacement varying device.
This embodiment is disadvantageous in that the difference between the target value and the approximate value obtained is big and also smooth starting of the prime mover cannot be made because the displacement of the pump is set maximum when it is stopped. (The displacement of the pump reaches its maximum value when the pilot pressure is lowest).